A spark plug is conventionally used in an internal combustion engine to ignite a fuel gas in a combustion chamber of the internal combustion engine. For example, there is known a spark plug of the type having a cylindrical insulator formed with an axial hole in the direction of an axis of the spark plug, a metal shell fixed around an outer circumference of the insulator and a center electrode partially inserted in a front end side of the axial hole. See Japanese Laid-Open Patent Publication No. 2009-26469; Japanese Laid-Open Patent Publication No. H09-264535; Japanese Laid-Open Patent Publication No. 2013-165016; and Japanese Laid-Open Patent Publication No. 2011-146130.
The insulator of the spark plug changes in temperature according to the status of the internal combustion engine. For example, the temperature of the insulator is increased by heat from combustion gas. The temperature of the insulator is decreased by introduction of fresh air into the combustion chamber. In this way, the insulator undergoes repeated temperature changes.
Herein, the insulator expands with increase in temperature and contracts with decrease in temperature. The insulator thus repeatedly expands and contracts according to repeated temperature changes. This can result in breakage of the insulator. In order to suppress such breakage of the insulator, it is conceivable to decrease the thickness of the insulator and thereby relieve stress caused to the insulator by expansion and contraction. When the thickness of the insulator is decreased, however, there arises a possibility of an unintentional discharge occurring between the center electrode and the metal shell through the insulator.
An advantage of the present invention is a spark plug capable of, while preventing an unintentional discharge from occurring between a center electrode and a metal shell through an insulator, improving the durability of the insulator against temperature changes.